Chemically strengthened glass is widely used for the production of electronic gadgets, displays, and the like. The total annual sale of thin strengthened glass is close to one billion dollars. Applying chemically strengthening glass to architectural and automotive glass will significantly increase this market.
The process of chemically strengthening glass is based on replacing smaller sodium ions inside of the glass with larger potassium ions. Because of size difference the compression stress is created. To facilitate the exchange of the sodium ions with the potassium ions, the glass is usually heated to the temperature below the strain point. The strain point is the point at which the particular glass has a dynamic viscosity of 1014.5 poises.
In general, hot salt baths are used to perform the ion exchange. To obtain a suitable depth of ion exchange, the process is on the order of hours, which very slow. This slow process is the primary reason why such large scale production of chemically strengthened architectural and automotive glass is expensive and has limited applications.
Also, architectural and automotive glass is usually soda lime glass. Soda lime glass is much cheaper than aluminosilicate glass. Aluminosilicate glass is usually used in the above-mentioned chemically strengthening processes. This is because, for soda lime glass, the energy activation of the potassium ion diffusion is much higher than in aluminosilicate glass and achieving any real necessary depths of ion exchange requires days of processing.
There are different methods for reducing the processing time of chemically strengthened class by modifying glass compositions and chemical strengthening salts. At best, these methods reduce the processing time down to the order of dozens of minutes or hours. In addition, these methods do not provide any substantial relief in the processing time for soda lime glass, which is cheaper than aluminosilicate glass.
U.S. Pat. No. 4,872,896 describes using a household microwave to reduce the total processing time for chemically strengthening a very small glass article.
Even if a reduction in the total processing time could have been achieved for some glass types using this method, no significant reduction in time can be achieved when soda lime glass is used. Also, such an approach cannot be used on an industrial scale for continuous in-line processing of large glass products, especially for chemically strengthening architectural and automotive products made from soda lime glass.
The present inventors are not aware of any method for reducing the processing time to chemically strengthen glass that does not require salt bathes and/or that is capable of being used for a large-scale continuous in-line processing as needed to meet the industry demand for chemically strengthened architectural and automotive glass.
Thus, there is a clear need in the art for a method that substantially reduces the time to perform an ion exchange process in a glass article on a large scale in a manner that is more effective with respect to processing time and expense.